Carboxy, carboalkoxy and carbamile substituted isonitrile radionuclide complexes

ABSTRACT

A coordination complex comprising a radionuclide selected from the class consisting of radioactive isotopes of Tc, Ru, Co, Pt and Re and an isonitrile ligand of the formula: 
     
         (CNX)R, 
    
     where X is a lower alkyl group having 1 to 4 carbon atoms, wherein R is selected from the group consisting of COOR 1  and CONR 2  R 3  where R 1  can be H, a pharmaceutically acceptable cation, or a substituted or unsubstituted alkyl group having 1 to 4 carbon atoms, R 2 , and R 3  can be H, or a substituted or unsubstituted alkyl group having 1 to 4 carbon atoms, and R 2  and R 3  can be the same or different is disclosed. Kits that can be used to form these complexes are also disclosed.

This invention was made with government support and the U.S. governmenthas certain rights in the invention.

U.S. Pat. No. 4,452,774 issued June 5, 1984 to Jones et al. disclosescoordination complexes of isonitrile ligands and radioactive metals, andis incorporated herein by reference. The coordination complexesdescribed therein are useful in visualizing cardiac tissue studying lungfunction, studying renal excretion, and for imaging bone marrow and thehepatobiliary system. These complexes are useful as diagnostic agentsfor labelling liposomes or vesicles and selected living cells.

We have now discovered that using isonitrile ligands having the formula:

    (CNX)R,                                                    (I)

where X is a lower alkyl group having 1 to 4 carbon atoms, R is selectedfrom the group consisting of COOR₁ and CONR² R³ ; where R¹ can be H, apharmaceutically acceptable cation, or a substituted or unsubstitutedalkyl group having 1 to 4 carbon atoms, R² and R³ can be H, or asubstituted or unsubstituted alkyl group having 1 to 4 carbon atoms, andR² and R³ can be the same or different, results in a complex having thegeneral advantages of the isonitrile radionuclide complexes of U.S. Pat.No. 4,452,774, but having generally superior properties with respect toliver clearance or lung clearance. Consequently, the complexes of thepresent invention can allow earlier imaging, and/or better imaging ofbodily tissues and organs than their corresponding parent compounds.

Thus, the present invention provides coordination complexes of Tc, Ru,Co, Pt or Re with the above isonitrile ligands of formula I.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scintigraphic image of a rabbit injected with a complex of^(99m) Tc with a carbomethoxyisopropylisonitrile ligand in accord withthe present invention.

FIG. 2 is a scintigraphic image of a pig injected with a complex of^(99m) Tc with a carbomethoxyisopropylisonitrile ligand in accord withthe present invention.

FIG. 3 is a comparison of counts per minute per pixel over time forcomplexes of ^(99m) Tc with methyl isonitrile ligands, and derivativesof methyl isonitrile of the present invention.

FIG. 4 is a comparison of counts per minute per pixel over time forcomplexes of ^(99m) Tc with ethyl isonitrile ligands and derivatives ofethyl isonitrile of the present invention.

FIG. 5 is a comparison of counts per minute per pixel over time forcomplexes of ^(99m) Tc with isopropyl isonitrile ligands and derivativesof isopropyl isonitrile of the present invention.

FIG. 6 is a scintigraphic image of a rabbit injected with a complex of^(99m) Tc with a carbomethoxyisopropylisonitrile ligand in accord withthe present invention.

FIG. 7 is a scintigraphic image of a rabbit injected with a complex of^(99m) Tc-2-carboethoxyethylisonitrile in accord with the presentinvention.

DESCRIPTION OF THE INVENTION

The present invention is directed to complexes of a radioactive metal(radionuclide) selected from the class consisting of Tc, Ru, Co, Pt andRe and a carboxylic acid, ester or amide derivative of a lower alkylisonitrile ligand (also herein referred to as a derived alkyl isonitrileligand). The isonitrile ligands useful in the present inventiondisclosed herein have the following general formula:

    (CNX)R,

where X is a lower alkyl group having 1 to 4 carbon atoms, and R isselected from the group consisting of COOR¹ and CONR² R³, where R¹ canbe H, a pharmaceutically acceptable cation, such as Na⁺, or asubstituted or unsubstituted alkyl group having 1 to 4 carbon atoms, R²and R³ can be H, or a substituted or unsubstituted alkyl group having 1to 4 carbon atoms, and R² and R³ can be the same or different.Preferably, X is a lower aklyl group having 1 to 3 carbons atoms.

Complexes using the present derived alkyl isonitrile ligands typicallyillustrate faster liver clearance or faster lung clearance when comparedto the corresponding parent isonitrile ligand. This generally enablescomplexes of the present invention to provide better and/or earlierimages than the corresponding parent isonitrile ligand. For example,with a heart agent, rapid liver clearance is desirable because it willincrease the contrast between the heart and the adjacent liver.Similarly, rapid lung clearance is desirable to improve the contrastbetween the heart and the lungs by reducing background "noise".Analogously, this rapid clearance enables earlier and/or better imagesof other tissues and/or organs which are blocked by background noisefrom the liver and/or lungs. Preferred complexes of the presentinvention provide optimal tissue uptake as well as faster liver and lungclearance.

Although complexes of this invention can be neutral or positively ornegatively charged, the class of lipophilic cationic complexes ispresently preferred. However, the complex cannot be too lipophilic andstill provide rapid liver and/or lung clearance. Therefore, it ispreferred that the total number of carbon atoms in each isonitrileligand is 12 or less. More preferably, the total number of carbon atomsin each ligand is 10 or less. Most preferably, the total number ofcarbon atoms in each ligand is 8 or less. When there are 4 carbon atomsin X, it is preferred that R has 5 or less carbon atoms, mostpreferably, R has 3 or less carbons atoms.

Because of the general availability of supplies of ^(99m) Tc in clinicallaboratories in the form of pertechnetate as well as the desirablehalf-life and gamma ray energy of this radionuclide, the complexes ofthe present invention preferably contain ^(99m) Tc. Additionally, thisabove-described general availability of supplies of pertechnetate makeit convenient to use kits for preparation of various complexes of ^(99m)Tc.

Therefore, the invention further comprises a kit for using apredetermined quantity of a radionuclide, e.g. ^(99m) Tc-pertechnetateto prepare a complex as stated above, said kit comprising the isonitrileligand of the formula (CNX)R and a reducing agent capable of reducingthe radioactive metal to form the coordination complex. The componentsof this kit are sterile and are packaged in a sterile, non-pyrogeniccontainer. Thus, they are ready for admixture with, for example, ^(99m)Tc pertechnetate to prepare complexes of the present invention.

For purposes of this invention, useful radionuclides are radioactivemetals having decay properties that are amendable for use as a tracer.

Counterions that maybe used in the invention include, for example, inthe case of cationic complexes, chloride, fluoride, bromide, iodide,hydroxide, sulfate or bisulfate, dihydrogen phosphate, fluoroborate,hexafluorophosphate, etc. Depending upon the particular radionuclideused, the valence state and other conditions for complexing, aparticular radioactive metal can have from one to eight isonitrileligands bonded thereto. As aforesaid, each isonitrile ligand is bondedto the radionuclide through the isonitrile carbon atom. Preferably, thecomplexes of this invention are kinetically inert, and hence stableproducts, However, the complexes need only be sufficiently stable forthe intended use.

The complexes of the present invention include mixed ligand complexeswherein at least one ligand is a derived alkyl isonitrile. The mixturecan include a mixture of isonitrile ligands, as well as other ligandswell known to those skilled in the art.

We presently believe that the complex of the present invention which isa homoleptic six coordinate (hexakis) cationic complex having theformula:

    [A(CNX)R.sub.6 ].sup.+

in which A is a monovalent radionuclide selected from Tc, or Re, and[(CNX)R] is a monodentate isonitrile ligand, is the most preferred. Asuitable counterion such as described above is also present.

The complexes of the present invention can easily be prepared byadmixing a salt of the radioactive metal and the isonitrile ligand ofthe present invention in the presence of a suitable reducing agent, ifrequired, in aqueous media at temperatures from room temperature toreflux temperature or even higher, and are obtainable and isolatable inhigh yield at both macro (carrier added, e.g. ^(99m) Tc) concentrationsand at tracer (no carried added, e.g. ^(99m) Tc) concentrations of lessthan 10⁻⁶ molar. In some cases the isonitrile ligand can itself act asthe reducing agent thus eliminating the need for an additional reducingagent. Suitable reducing agents, when required or desired, are wellknown to those skilled in the art. The reaction is generally completewithin 2 hours, depending upon the identity of the particular reagentsemployed. The radiolabelled complex is made in the same way as thecorresponding non-radioactive isonitrile complex by simply substitutingthe desired radionuclide for the corresponding non-radioactive elementin the starting materials, except in the case of technetium because alltechnetium isotopes are radioactive.

In the case of technetium such as, for example ^(99m) Tc or ^(99m) Tc, acomplex in accord with this invention is preferably made by mixingpertechnetate (Tc⁺⁷) with the desired isonitrile in an aqueous medium,then adding to the reaction mixture an appropriate reducing agentcapable of reducing the technetium. Suitable reducing agents includealkali metal dithionites, stannous salts, sodium borohydride, and otherswell known to the skilled artisan.

The derived alkyl isonitrile technetium complexes of this invention canalso be prepared from preformed technetium complexes having oxidationstates for technetium of, for instance, +3, +4, or +5, by treating thesepreformed complexes with an excess of derived alkyl isonitrile ligandsunder suitable conditions. For example, the technetium-derived alkylisonitrile complex can also be prepared by reacting the desiredisonitrile ligand with the hexakis-thiourea complex of Tc⁺³ or with atechnetium-glucoheptonate complex, or the like.

Kits in accord with the present invention comprise a quantity of areducing agent for reducing a preselected radionuclide. Preferably, suchkits contain a predetermined quantity of a derived alkyl isonitrileligand and a predetermined quantity of a reducing agent capable ofreducing a predetermined quantity of a preselected radionuclide. Theisonitrile ligand and reducing agent are generally provided in sealed,sterilized containers.

In one embodiment of the invention, a kit for use in making thecomplexes of the present invention from a supply of ^(99m) Tc such asthe pertechnetate solution in isotonic saline available in most clinicallaboratories includes the desired quantity of a selected derived alkylisonitrile ligand to react with a selected quantity of pertechnetate,and a reducing agent such as sodium dithionite or stannous chloride inan amount sufficient to reduce the selected quantity of pertechnetate toform the desired complex.

A molar excess of the derived alkyl isonitrile ligand, typically 600%molar excess or more, and an excess of reducing agent, can be used inthe complexing reaction to ensure maximum yield of the desired complexfrom the technetium. Following the reaction, the desired complex can beseparated from the reaction mixture, if required, by crystallization orprecipitation or by conventional chromatography or ion exchangechromatography.

The following specific examples are intended to illustrate more fullythe nature of the present invention. They should not be construed aslimiting its scope, which is set forth in the claims.

EXAMPLE 1 Methylisocyanoacetate

To a solution of glycine methylester-hydrochlodride (10.13 g, 0.08 mol)in formic acid (95%, 30 ml) was added a solution of sodium formate informic acid (5.5 g in 10 ml). The solution was heated to dissolve thesalts and then stirred for 2 hours at 40° C. The white precipitate(NaCl) was removed by filtration through Celite™ 545. To the filtratewas added acetic anyhydride (30 ml) and formic acid (50 ml) in a 300 mlround-bottomed flask equipped with a condenser and magnetic stirrer. Itwas stirred for 2 hours, when the initial reaction was over the mixturewas refluxed for 40 hours. And a further aliquot (20 ml) of the aceticanyhydride formic acid mixture (1:2 v/v) and refluxed for 16 hours. Theabove procedure was repeated once more. The solvents were removed invacuo at room temperature. The product methylformamidoacetate wasseparated from the residue by vacuum distillation (142°-145° C., 15 mmHg). The recovered yield was 7.12 g (76%).

¹ H NMR: δ3.7 (s 3H, Me); δ4.0 (d J=4 Hz --CH₂); δ6.4 (vb 1H --NH--);δ8.03 (s,1H --CHO).

The formamide, vide supra, (7.11 g, 0.06 mol) was dissolved in CH₂ Cl₂(100 ml) and placed in a 500 ml triple-necked round-bottomed flask. Thesolution was purged with argon and cooled to -30° C. To the flask wasadded Me₃ N (Me=methyl) (35 ml) dissolved in CH₂ Cl₂ (75 ml). Themixture was cooled and stirred while adding dropwise a solution oftrichloromethyl chloroformate (diphosgene) (6.01 g 0.030 moles) in CH₂Cl₂ (50 ml). The solution turned color from tan to brown. The mixturewas allowed to warm slowly to room temperature and then heated to refluxfor 30 minutes. The reaction mixture was treated with ammonium hydroxide(30%, 100 ml). The CH₂ Cl₂ layer was separated. The aqueous layer wasextracted three times with CH₂ Cl₂ (50 ml) and the extracts combined.The CH₂ Cl₂ extract was dried over sodium sulfite. The volume wasreduced under vacuum and the product, methylisocyanoacetate, separatedby vacuum distillation (64°-65° C., 10 mm of Hg). Yield, 1 ml.

¹ H NMR: δ3.83 (s 3H-OMe); δ4.23 (s 2H --CH₂ --). IR: V_(NC) 2182 cm⁻¹ ;V_(CO) 1782 cm⁻¹.

EXAMPLE 2 Hexakis carbomethoxymethylisonitrile Technetium (I)Hexafluorophosphate

Hexakis(thiourea-S)technetium (III) (Abrams, M. J., et al, Inorg. Chem.,23:3284-3288 (1984); Abrams, M. J., et al, J. Lab. Comp. Radiopharm.,14:1596 (1982)) tetrafluoroborate (0.42 g) was dissolved in MeOH (50ml), the system purged with argon, and methylisocyanoacetate (150 ul)was added to the clear red solution. The mixture was heated to reflux(30 minutes), and the volume reduced to ca. 10 ml in vacuo. The residuewas titrated with a solution of NH₄ PF₆ in MeOH (5 ml). Addition of Et₂O (Et=ethyl) (10 ml) caused the crude solid to separate.

Recrystallization from acetone/ether gave white crystals (0.24 g,43.8%).

¹ H NMR: δ3.7 (s 3H, ml); δ4.58 (b, s 2H --CH₂ --). FAB(+)MS: M/Z 693.

A Kit For The Preparation Of No Carrier Added (NCA) Hexakiscarbomethoxymethylisonitrile Technetium Cation From Generator Eluant(^(99m) _(TcO) ₄ ⁻)

To a 5 ml serum vial containing Na₂ S₂ O₄ (4-5 mg) was added EtOH (0.5ml) and methylisocyanoacetate (20 ul) (uls microliters). The vial wassealed and ^(99m) Tc pertechnetate in normal saline generator eluant(0.5 ml) injected into the vial. The vial and contents were shaken for 5seconds and heated to 60° C. for 1 hour. HPLC and reversed phase TLCshowed that the yield of the complex was 93%.

EXAMPLE 3 t-Butylisocyanoacetate

This ester was prepared similarly to the methylisocyanoacetate. Theglycine t-butylesterhydrochloride was prepared from glycine suspended int-ButOH (But=butyl) and treated with anhydrous HCl. The amineesterhydrochloride was recrystallized from methylethyl ketone/hexane.The intermediate t-butylformamidoacetate was distilled at 115°-120° C.,1 mm of Hg.

¹ H NMR: δ1.67 (s 9H C(CH₃)₃); δ4:03 (d, 2H J+4 HZ --CH₂ --); δ6.67 (bs,1H, --NH--); δ8.08 (s, 1H, --CHO). t-butylisocyanoacetate distilled at54°-56° C., 0.4 mm Hg ¹ H NMR: δ1.67 (s, 9H, --C(CH₃)₃); δ4.05 (s, 2H,--CH₂ --). IR: V_(NC), 2162 cm⁻¹ ; V_(CO), 1762 cm⁻¹.

Hexakis Carbo-t-butoxymethylisonitrile Technetium (I) Tetrafluoroborate

This salt was made in a similar manner to thehexakis-carbomethoxymethylisonitrile technetium derivative except thatt-ButOH was used as a solvent and the product chromatographed on neutralalumina with CH₂ Cl₂. Concentration of the eluant and the addition ofhexane ether (1:1 v/v) gave white crystals on cooling (5° C.).

FAB(+) MS: M/Z 945.

The kit for the NCA preparation of ^(99m) TC-hexakiscarbo-t-butoxymethylisonitrile technetium was similar to that forhexakis carbomethoxymethylisonitrile technetium except that the vialcontents were heated at 45° C. for 75 minutes for >95% yield (HPLC andTLC).

EXAMPLE 4 Methyl-2-aminoisobutyrate hydrochloride

The methyl ester hydrochloride was made by the treatment of 2aminobutyric acid with anhydrous HCl in MeOH. M.pt.: 157°-158° C.

Methyl-2-formamidoisobutyrate

The methyl-2-aminoisobutyrate hydrochloride was converted byN-formylation using the formic acid, acetic anhydride procedure in 60%yield.

B.pt. 86°-89° C., 0.8 mm Hg.

1H NMR: δ1.6 (s, 6H C(CH₃)₂); δ3.73 (s, 3H, OMe); δ70 (s, b 1H, NH);δ8.28 (s, 1H, CHO).

Methyl-2-isocyanoisobutyrate was obtained by dehydration of theformamide with diphosgene.

B.pt.: 70°-71° C., 26 mm Hg.

1H NMR: δ1.68 (s, 6H C(CH₃)₂); δ3.83 (2, 3H, OMe). IR: V_(NC) 2141 cm⁻¹; V_(CO) 1752 cm⁻¹.

Kit Formulations For The NCA Preparation Of ^(99m)Tc-Hexakis-1-carbomethoxyisopropylisonitrile Technetium (I)

(a) To a 5 ml serum vial containing Na₂ S₂ O₄ (5.5 mg) was added EtOH(0.5 ml) and methyl-2-isocyanoisobutyrate (20 ul). The vial was sealedand ^(99m) Tc pertechnetate normal saline generator eluant (0.5 ml)injected into the vial. The vial was heated (60° C.) for 50 minutes,HPLC and TLC (reversed phase) showed the yield of the complex was 97%.

(b) To a 5 ml serum vial was added EtOH (0.5 ml) andmethyl-2-isocyanoisobutyrate (20 ul). The vial was sealed. It was theninjected with a reconstituted technetium ^(99m) Tc glucoheptate sodiumkit (Glucoscan™) (0.5 ml) and allowed to stand 2 hours at 23° C. HPLCand TLC showed that the yield of the complex was 84%.

(c) To a 5 ml serum vial was added stannous tartrate (2.15 mg), MeOH(0.8 ml), and methyl-2-isocyanoisobutyrate (20 ul). The vial was sealedand injected with ^(99m) Tc-pertechnetate normal saline generator eluant(0.2 ml) and allowed to stand for 60 minutes. The yield was 70%.

EXAMPLE 5 Isocyanoacetamide

Gaseous NH₃ was bubbled through a solution of methylisocyanoacetate (6.2g) and MeOH (10 ml) for 15 minutes. The contents were stirred for 15minutes, then concentrated. The addition of Et₂ O cause theisocyanoacetamide to separate as white crystals which were collected anddried in vacuo. The yield was 1.4 g (25%)

Anal Calcd for C₃ H₄ N₂ O₂ C,54.66, H, 4.70; N, 1822; Found C, 53.49; H,4.70; N 1775.

IR: V_(CN) 21114 cm⁻¹, V_(CO) 1650 cm⁻¹.

Kit For The Preparation Of NCA ^(99m) Tc Hexakis-carbomethoxyisonitrileTechnetium (I)

To a 5 ml septum vial was added Na₂ S₂ O₄ (5.5 mg) and isocyanoacetamide(11 mg). The vial was sealed prior to the injection (1 ml) of generatoreluant (^(99m) Tc pertechnetate in normal saline). The contents wereheated at 100° C. for 1 minute. The yield was 97% (HPLC).

Preparation Of NCA Hydrolysed Hexakis-carbomethoxymethylisonitrileTechnetium

To a ^(99m) Tc reconstituted kit of NCA hexakiscarbomethoxymethylisonitrile technetium (I) was added aqueous sodiumhydroxide (0.5 ml, 0.5M) and the solution was heated at 60° C. for 15minutes. The vial contents were neutralized with dilute HCl (0.5M).

EXAMPLE 6 Ethyl-3-aminopropanoate hydrochloride

The ethyl-3-aminopropanoate hydrochloride was made by the treatment of 3aminopropanic acid with anhydrous HCl in EtOH.

Ethyl-3-formamidopropanoate

The ethyl-3-aminopropanoate hydrochloride was converted by N-formylationusing the formic acid, acetic anhydride procedure in 59% yield.

B.pt. 131°-136° C., 15 mm Hg.

Ethyl-3-isocyanopropanoate was obtained by dehydration of the formamidewith diphosgene. The yield was 65%.

B.pt.: 70°-71° C., 19 mm Hg.

Kit Formulation For The NCA Preparation Of ^(99m)Tc-Hexakis-2-carboethoxyethylisonitrile Technetium (I)

To a 5 ml serum vial containing Na₂ S₂ O₄ (5.7 mg) was added EtOH (0.5ml) and ethyl-3-isocyanopropanoate (20 ul). The vial was sealed and^(99m) Tc pertechnetate normal saline generator eluant (0.5 ml) injectedinto the vial. The vial was heated (60° C.) for 50 minutes, HPLC and TLC(reversed phase) showed the yield of the complex was better than 98%.

EXAMPLE 7 Preparation of Injection Media

(a) The NCA preparations of the ^(99m) Tc complexes can be diluted withnormal saline to 25% EtOH, and then filtered (Sartorius Minisart™ NML).Suitable aliquots of these solutions can be used for biologicalevaluation.

(b) For those situations which required a pure preparation free of thereactants, the following procedure can be used. After the reconstitutionwith ^(99m) Tc the contents of the kit are diluted 4 to 1 with water andthen eluted through a prewetted Water Associates Sep-Pak™ C₁₈ cartridge.The radiolabel is retained in the cartridge, and is washed with saline(5 ml) followed by ethanol/saline (10 ml, 45/55, v/v) and is eluted withmixture of 85% EtOH, 5% ammonium acetate (0.5M) and 10% saline (0.15M)(1.5 ml). The eluant is diluted with saline (0.15M) and filtered througha 0.2 micron Sartorius Minisart™ NML filter to yield a sterile, pyrogenfree solution suitable for biological evaluation.

Extraction of the Product

This step may be used with the above syntheses to provide a pure sampleof the isonitrile complex freed from the other materials in thesyntheses.

The solution was transferred to a separatory funnel (50 ml) and twiceextracted with methylene chloride (3 ml). The organic phase was twicewashed with isotonic saline (5 ml) and then transferred to a siliconizedround-bottomed flask (50 ml) fitted with a vacuum adaptor. The solventwas removed in vacuo, aided by heating with an infra-red lamp. The flaskwas washed first by addition of ethanol (100 ul) followed by saline (1ml). The solution was then ready for administration to animals afterassay by HPLC, the complex being in the form of a solution in aphysiologically acceptable non-toxic carrier.

EXAMPLE 8 Scintigrapic Imaging Procedure in Animals

Rabbits

Two rabbits were anesthetized with sodium pentobarbital and injected viaan ear vein with 3-8 mCi of ^(99m) Tc-isonitrile complex. One rabbit wasimaged using a gamma camera equipped with a pinhole collimatorpositioned directly over the chest. Three minute static images wereacquired at 5, 10, 20, 20, 40, 50 and 60 minutes post injection. Asecond rabbit was used to investigate the relative whole bodydistribution of activity for 60 minutes post-injection. Immediatelypost-injection, 300 thousand count images were acquired in the followingsequence of projections: chest, anterior heart, whole body, anteriorheart, left lateral whole body, anterior heart, lateral heart, anteriorheart.

Swine

Swine were sedated with intramuscular injections of Ketamine andacepromazine, and anesthetized with sodium pentobarbital. The animalswere positioned for anterior image acquisition over the chest under alow energy, high resolution collimator and injected with 10-12 mCi^(99m) Tc-isonitrile complex via a leg vein. The liver was covered witha lead shield, and three minute images were acquired at 5, 10, 20, 30,40, 50 and 60 minutes post-injection.

Dogs

After anesthetization with sodium pentobarbital, dogs were injected with25 mCi of ^(99m) Tc and positioned for anterior chest image acquisitionusing a pinhole collimator. Again, three minute images were obtained at5, 10, 20, 30, 40, 50 and 60 minutes post-injection.

Representative scintigraphic images from 5 to 60 minutes post-injectionfor rabbits and pigs are shown in FIGS. 1 and 2, respectively.

EXAMPLE 9

Rabbits were anesthetized with Ketamine and Nembutal™ and positionedbeneath a gamma camera to allow continuous collection of data includinga chest and abdomen view. 1 mCi of the test agent as specified below wasinjected via an ear vein and data collected continuously at 60 secondintervals for 60 minutes. Curves were generated by drawing regions ofinterest over selected tissues and normalizing to counts per minute perpixel.

The following complexes of ^(99m) Tc with methyl isonitrile ligands wereinjected into rabbits: CNCH₂ R where R was --N, --COOCH₃, --COOC₂ H₅,--COO^(n) C₃ H₇, --COO^(t) C₄ H₉ and --CONH₂. The effect of thesesubstitutions on the clearance curve of the complex through the liver isset forth below in Table 1.

                  TABLE 1                                                         ______________________________________                                        Liver Activity for Complexes of .sup.99m Tc                                   Formed with Ligands of the Type CNCH.sub.2 R                                  Ligand         Liver Activity                                                 CNCH.sub.2 R   t.sub.max (min)                                                                         t.sub.1/2  (min)                                     ______________________________________                                        R = --H        14        88                                                   --COOCH.sub.3  1         37                                                   --COOC.sub.2 H.sub.5                                                                         3         12                                                   --COO.sup.n C.sub.3 H.sub.7                                                                  2         11                                                   --COO.sup.t C.sub.4 H.sub.9                                                                  5         25                                                   --CONH.sub.2   1         13                                                   ______________________________________                                    

Liver activity for ethyl isonitrile and some of its derivatives aredescribed in Table 2. While Table 3 shows the liver activity forisopropylisonitrile and some of its derivatives.

                  TABLE 2                                                         ______________________________________                                        Liver Activity for the Complexes of                                           .sup.99m Tc Formed with Ligands of the                                        Type CNCH(R)CH.sub.3 or CNCH.sub.2 CH.sub.2 R                                               Liver Activity                                                  Ligand          t.sub.max (min)                                                                         t.sub.1/2 (min)                                     ______________________________________                                        a) CNCH(R)CH.sub.3 :                                                          R = --H         2         270                                                 --COOCH.sub.3   2         45                                                  --COOC.sub.2 H.sub.5                                                                          4         12                                                  b) CNCH.sub.2 CH.sub.2 R                                                      R = --H         2         270                                                 --COOCH.sub.3   7         85                                                  --COOC.sub.2 H.sub.5                                                                          2         17                                                  ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Liver Activity for the Complexes of .sup.99m Tc Formed                        with Ligands of the Type CNC(CH.sub.3).sub.2 R                                               Liver Activity                                                 Ligand           t.sub.max (min)                                                                         t.sub.1/2                                          ______________________________________                                        CNC(CH.sub.3).sub.2 R                                                         R = --H          5         98                                                 --COOCH.sub.3    3         35                                                 --COOC.sub.2 H.sub.5                                                                           5         22                                                 --COO.sup.n C.sub.3 H.sub.7                                                                    2         39                                                 --COOH           1          6                                                 --CONH.sub.2     2         44                                                 ______________________________________                                    

The data clearly shows that the clearance of the derived compounds ofthe present invention is substantially faster than that of thecorresponding parent compound. All the data in the above three tablesare from dynamic camera studies in rabbits by the procedure describedabove. The time of maximum activity in the liver post injection is "^(t)max", while "t1/2" is the apparent half life of the clearance curve from^(t) max. The latter data is an estimate based on data collected to 60minutes after administration. This assumes a simple exponential curvewith time, although more extensive studies might indicate morecomponents are actually present.

FIG. 3 is a comparison of counts per minute per pixel (pix) over time inminutes for complexes of ^(99m) Tc with isonitrile ligands of theformula CN(CH₂)R, where R is:

(a) --H,

(b) --COOCH₃

(c) --COOC₂ H₅ ;

(d) --COO^(n) C₃ H₇ ; and

(e) --COO^(t) C₄ H₉.

The complexes were prepared, injected and imaged as described above.

FIG. 4 is a comparison of counts per minute per pixel over time(minutes) for complexes of ^(99m) Tc with isonitrile ligands of theformula:

(a) CNCH₂ CH₃

(b) CNCH(R)CH₃, where R is --COOCH₃ ;

(c) CNCH(R)CH₃, where R is --COOC₂ H₅ ;

(d) CNCH₂ CH₂ (R), where R is --COOCH₃ ;

(e) CNCH₂ CH₂ (R), where R is --COOC₂ H₅.

FIG. 5 is a comparison of counts per minute per pixel over time(minutes) for complexes of ^(99m) Tc with isonitrile ligands of theformula CNC(CH₃)₂ R, where R is:

(a) --H;

(b) --COOCH₃

(c) --COOC₂ H₅ ;

(d) --COO^(n) C₃ H₇ ;

(e) --COOH; and

(f) --CONH₂.

These figures show that complexes containing the derived alkylisonitrile ligand have faster liver and/or lung clearance than thecorresponding parent compound. Consequently, the complexes of thepresent invention can allow earlier imaging and/or better imaging ofbodily tissues and organs.

For example, coordination complexes of the present invention containingligands of carbomethoxyisopropylisonitrile or2-carboethoxyethylisonitrile appear to be particularly useful forimaging the heart. Because of rapid lung clearance there is very good toexcellent resolution of the myocardium at five minutes after injectionof complexes containing ligands of carbomethoxyisopropylisonitrile inrabbits when imaged. Although the liver uptake results in backgroundnoise that initially prevents clear visualization of the apex of theheart, by 20 minutes after injection, there has been sufficient liverclearance to allow excellent resolution of the apex. FIG. 6(a) is ascintigraphic image of a rabbit injected with a complex of ^(99m)Tc-carboethoxyisopropylisonitrile at 5 minutes after injection, whileFIG. 6(b) is a scintigraphic image of the same rabbit at 15 minutesafter injection. FIG. 7(a) is a scintigraphic image of a rabbit injectedwith a complex of ^(99m) Tc-2-carboethoxyethylisonitrile at 1, 10, 15and 20 minutes post injection. FIG. 7 (b) is a scintigraphic image of arabbit injected with the above complex at 31 minutes post-injection. Theheart is at (A), the gall bladder at (B) and the site of injection at(C). The interpretation of these images is known to the person ofordinary skill in this field. While reading of these images is somewhatqualitative, the above described ^(99m) Tc-carboethoxyethylisonitrilecompound has resulted in excellent images in both animal and humantests.

This invention has been described in detail including the preferredembodiments thereof. However, it will be appreciated that those skilledin the art, upon consideration of this disclosure, may makemodifications and improvements within the spirit and scope of thisinvention.

We claim:
 1. A coordination complex of a lower alkyl isonitrile ligandof the formula:

    CN--X--R,

where X is a lower alkyl group having 1 to 4 carbon atoms, wherein R isselected from the group consisting of COOR¹ and CONR² R³ where R¹ can beH, a pharmaceutically acceptable cation, or a substituted orunsubstituted alkyl group having 1 to 4 carbon atoms, R², and R³ can beH, or a substituted or unsubstituted alkyl group having 1 to 4 carbonatoms, and R² and R³ can be the same or different; and a radioactivemetal selected from the class consisting of radioactive isotopes of Tc,Ru, Co, Pt and Re.
 2. The coordination complex of claim 1, wherein X isa lower alkyl group having 1 to 3 carbon atoms.
 3. The coordinationcomplex of claim 1, wherein the total number of carbon atoms in eachligand is 12 or less.
 4. The coordination complex of claim 3, whereinthe total number of carbon atoms in each ligand is 10 or less.
 5. Thecoordination complex of claim 3, wherein the total number of carbonatoms in each ligand is 8 or less.
 6. The coordination complex of claim1, wherein the isonitrile ligand is selected from the group consistingof CNCH₂ COOH, CNCH₂ COOCH₃, CNCH₂ COOC₂ H₅, CNCH₂ COOC₃ H₇, CNCH₂ COOC₄H₉ and CNCH₂ CONH₂.
 7. The coordination complex of claim 1, wherein theisonitrile ligand is selected from the group consisting of CNC₃ H₆ COOH,CNC₃ H₆ COOCH₃, CNC₃ H₆ COOC₂ H₅, CNC₃ H₆ COOC₃ H₇, and CNC₃ H₆ CONH₂.8. The coordination complex of claim 1, wherein the isonitrile ligand isselected from the group consisting of CNC(CH₃)₂ COOH, CNC(CH₃)₂ COOCH₃,CNC(CH₃)₂ COOC₂ H₅, CNC(CH₃)₂ COOC₃ H₇ and CNC(CH₃)₂ CONH₂.
 9. Thecoordination complex of claim 1, wherein the isonitrile ligand is CNC₃H₆ COOCH₃.
 10. The coordination complex of claim 1, wherein theisonitrile ligand is CNC(CH₃)₂ COOCH₃.
 11. The coordination complex ofclaim 1, wherein the isonitrile ligand is CNC₂ H₄ COOC₂ H₅.
 12. Thecoordination complex of claim 1, wherein the isonitrile ligand is CNCH₂CH₂ COOC₂ H₅.
 13. The coordination complex of claim 1, wherein X of theisonitrile ligand has 4 carbon atoms, and R has 5 or less carbon atoms.14. The coordination complex of claim 13, wherein R has 3 or less carbonatoms.
 15. A complex as claimed in claim 1 in which said metal is Tc.16. A complex as claimed in claim 1 in which said metal is Re.
 17. Acomplex as claimed in claim 1 wherein each coordinate position of saidradioactive metal is filled by an isonitrile ligand.
 18. A complex asclaimed in claim 1 wherein the complex is formed from a mixture ofisonitrile ligands.
 19. A complex as claimed in claim 1 wherein saidcomplex is a cationic lipophilic complex.
 20. An isonitrile complexhaving the formula:

    [A(CN--X--R).sub.6 ].sup.+

wherein A is a radionuclide selected from the group consisting of Tc andRe, where X is a lower alkyl group having 1 to 4 carbon atoms, and R isselected from the group consisting of COOR¹ and CONR² R³ where R¹ can beH, a pharmaceutically acceptable cation, or a substituted orunsubstituted alkyl group having 1 to 4 carbon atoms, R², and R³ can beH, or a substituted or unsubstituted alkyl group having 1 to 4 carbonatoms, and R² and R³ can be the same or different.
 21. A hexakis complexof a technetium radionuclide and an alkyl isonitrile ligand of theformula CNC₃ H₆ COOCH₃.
 22. A hexakis complex of a technetiumradionuclide and an alkyl isonitrile ligand of the formula CNC(CH₃)₂COOCH₃.
 23. A hexakis complex of a technetium radionuclide and an alkylisonitrile ligand of the formula CNC₂ H₄ COOC₂ H₅.
 24. A hexakis complexof a technetium radionuclide and an alkyl isonitrile ligand of theformula CNCH₂ CH₂ COOC₂ H₅.
 25. A method for imaging body tissuescomprising administering to a mammal a radiopharmaceutical compositioncomprising a coordination complex of an isonitrile ligand of claim 1 and^(99m) Tc, and detecting the localization of such complex in the bodytissues by a means for detecting radiation.
 26. The method of claim 25wherein the means for detecting radiation is a gamma camera.
 27. Themethod of claim 25 wherein said complex is ^(99m)Tc-hexakis-carbomethoxyisopropylisonitrile.
 28. The method of claim 25wherein said complex is ^(99m) Tc-hexakis-2-carboethoxyethylisonitrile.